Coating of solid substrates with magnetically propelled particles

ABSTRACT

Particulate material is coated onto a solid substrate surface by exposing the surface in a confined volume containing small magnet elements mixed with the particulate material, and establishing within an effective distance of the confined volume, a magnetic field varying in direction with time. The magnetic field is of sufficient intensity to propel the magnet elements and the particulate material at a velocity which causes the mixture to impinge upon the surface and the particulate material to adhere thereto as a layer.

United States Patent 1191 Lov ness 1 July 1,1975

[ COATING OF SOLID SUBSTRATES WITH MAGNETICALLY PROPELLED PARTICLES [75] Inventor: William R. Lovness, West Saint Paul, Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

221 Filed: June 25, 1973 211 Appl. No.: 373,028

[52] US. Cl. 428/329; 117/109; 117/234; 117/D1G. 1; 117/D1G. 8', 118/620; 259/99; 259/DlG, 46; 117/13l.17 [51] Int. Cl B44d 1/094 [58] Field of Search 117/932, 109, DIG. l, l17/D1G. 8, 234, 131, 17.5, 17; 118/57, 76,

[56] References Cited UNITED STATES PATENTS 2,735,231 2/1956 Simjian 51/7 2,880,554 4/1959 Simjian 51/7 3,219,318 11/1965 Hershler 259/1 3,318,284 5/1967 Hojo et a1, 117/175 3,423,880 l/l969 Hershler 51/7 3,439,899 4/1969 Hershler 3,455,276 7/1969 Anderson 117/175 FOREIGN PATENTS OR APPLICATIONS 1,218,287 6/1966 Germany 117/175 Primary ExaminerRalph Husack Assistant Examiner1ohn 1-1. Newsome Attorney, Agent, or Firm-Alexander, Sell, Steldt and Delahunt [57] ABSTRACT 7 Claims, 2 Drawing Figures COATING OF SOLID SUBSTRATES WITH MAGNETICALLY PROPELLED PARTICLES BACKGROUND OF THE INVENTION This invention relates to the coating of solid substrates with various materials. More particularly. the present invention is directed to the coating of various particulate materials on the surface of solid substrates by utilizing particles propelled by magnetic forces.

DESCRIPTION OF THE PRIOR ART Numerous techniques are known in the art for coating solid articles. An article may be coated to modify its surface properties such as corrosion resistance. electrical contact resistance. reflectivity. color. abrasion resistance. solderability. coefficient of friction. etc.

The common methods ofcoating are chemical reduction. electroplating. spraying. hot dipping. mechanical plating and vacuum metallizing. Chemical reduction requires stringent temperature control. generally produces noxious fumes. and is not economical. The quality of chemically reduced deposits is inferior to that of either electroplated metal deposits or vacuummetalized deposits with respect to dimensional control. durability and reflectivity. The processing temperatures involved with chemical reduction practices generally exceed the heat-distortion point of most plastics. thereby precluding the use of this process for coating plastics.

Electroplating is limited in the number of metals which can be plated. causes hydrogen embrittlement and has the disadvantage of requiring a conductive substrate. thereby also precluding the coating of plastic by this technique unless the plastic substrate is provided with a conductive surface.

Metal spraying. applicable primarily for heavy deposits. produces a coating which is porous. dimensionally nonuniform. and usually requires thermal treatment to improve adherence. The finish of sprayed metal coatings is rough and unattractive.

Commercial hot-dipped coatings are limited to low melting metals such as zinc. tin. lead and aluminum. Additionally. hot dipping requires an extremely clean. greaseand oxidefree surface to obtain a uniform adherent coating.

The process of mechanical plating has been known for perhaps a quarter of a century. The broad principles ofthe process are well known; see. e.g.. British Pat. No. 534.888. US. Pat. Nos. 2.689.808. and Re. 23.861, and other publications. The process is typically carried out by placing in a tumbling barrel metallic parts to be plated. plating metals in the form of minute malleable particles. impact media such as glass beads and cullet. water. and. optionally. a chemical promoter. As the tumbling barrel is rotated. the plating metal particles are hammered against the surface of the metallic parts to be plated. the impact media and the parts themselves serving to flatten the metal particles into a continuous coat. Mechanical plating may produce adequate results but is generally limited to only a few metals such as tin. zinc. cadmium. and brass.

Mechanical plating may also be accomplished by projecting an air borne mixture of coatable particles and hard peening particles onto a substrate causing hammering of the coatable particles on the surface as a layer. Such a process is limited by the trajectory of the stream ofthe air borne mixture to relatively flat and uniformly shaped substrates.

SUMMARY OF THE PRESENT INVENTION In accordance with the process of the invention. particulate material is plated on a substrate surface by exposing the surface in a confined volume containing small magnet elements mixed with the particulate material and establishing within an effective distance of the confined volume a magnetic field varying in direction with time.

Without being bound by any theory or scientific explanation of precisely how the present invention functions. it is believed that the magnetic field imparts motion to the magnet elements which in turn imparts a motion to the particulate material mixed therewith. These materials then impinge upon the surface of the substrate in a sufficient amount and with sufficient force to clean the surface and hammer the particulate material thereon to form a uniform coating. Microscopic examination of the coated surface. prior to completion of the coating. indeed reveals a multitude of flattened particles adhered to the substrate surface. in

some cases.

The present invention provides a coating process which permits simple or very complex shaped articles of plastic. metal. or any hard material to be coated with any of a variety of materials including plastics. metals. inorganic materials and others. The process utilizes simple economical apparatus and produces no undesirable waste products which require removal or disposal. The process. which requires no toxic chemicals. does not utilize molten metal and therefore eliminating the danger of burns and fires caused thereby. The process can be used to coat fragile articles. very complex articles. and articles not capable of being coated by conventional techniques. The process provides uniform coatings of good quality from very thin to very thick. with no modification thereof merely by continuing coating for the appropriate time. No hydrogen embrittlement is produced by the process of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view of a coating apparatus in accordance with the invention; and

FIG. 2 is a vertical section view. taken at lines 2-2 of the apparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION As shown in FIGS. 1 and 2. the apparatus utilized for coating in accordance with the invention is comprised ofa magnetic field generating device I0 capable of producing a magnetic field which varies in direction with time. small magnet elements 11 capable of being moved by the magnetic field. and particulate material 12 which is to be coated. A container 13 confines the mixture of magnet elements particulate material 12, and substrate 14 being coated. in a predetermined volume. (It should be noted that magnetic field generating device 10 is shown as a solid annulus merely for purposes of illustration. and it will actually have other parts such as wires. cores. etc. as will be apparent from the description which follows.)

The magnetic field may be generated by means of osillators. oscillator/amplifier combinations. solid-state pulsating devices. motor generators. and mechanical vibrators. The magnetic field may also be provided by 7 means of air or metal core coils. stator devices or the like. The preferred device for generating the magnetic field is capable of generating a rotating magnetic field. With such a device. the field which is generated rotates about a central axis defined by the device itself.

A preferred device for generating such a rotating magnetic field. is described in assignees copending application to Lovness and Feldhaus. Ser. No. 334.000. filed Feb. 20. 1973. the disclosure of which is incorpo rated herein by reference. This device has at least four overlapping electrical coils arranged in a generally cir cular pattern of opposed pairs and energized by two or more oubof-phase sources of alternating current so that opposed coils are of opposite polarity and of the same phase. A rudimentary version of this type of field generator device is the stator of a two pole alternating current electric motor.

The container or surface for confining the magnet elements and particulate coating material within a predetermined area should be formed of a non-magnetic ma terial such as glass. synthetic organic plastics. for example. polytetrafluoroethylene (e.g.. Teflon"). polyethylene. polypropylene. and the like. ceramics. nonmagnetic metals such as stainless steels. bronze. lead. etc.

Any one of a variety of particulate materials of varying degrees of hardness and shape is contemplated for use us the coating material of the present invention. For the most part. the coating materials are metal powders but other materials have also been found suitable for coating. Illustrative of metal powders which may be coated are aluminum. iron. lead. zinc. cadmium. copper. indium. tantalum. chromium. magnesium. nickel. tungsten. silver. and gold. Illustrative metal alloy powders which have been found useful for coating include stainless steels. aluminum/zinc alloys. and tin/lead alloys. Non-metal powders found useful for coating in the present invention include graphite. molybdenum disulfide. and organic resins such as polytetrafiuoroethylene and polyvinyl chloride.

The shape of the particulate material being coated need not be in any particular form since it has been found virtually all shapes will provide a suitable coating. e.g.. round. flake. etc. The particulate material may range in size from 0.1 micron or less in maximum dimension to several hundred microns or more; preferably the particle size is within the range of 0.5 to 50 microns. The mass of each magnet element is preferably at least twice the mass of each individual fragment of particulate material being coated or else very large magnetic forces are required to provide uniform and permanent coatings.

As previously mentioned. the process of the inven tion utilizes small magnet elements. each of which is an individual minute permanent magnet and hence susceptible to the influence of a moving magnetic field. Such elements include gamma iron oxide (F6 03). hard barium ferrite. (Ba0.6Ee particulate aluminumnickelcobalt alloys. or mixtures thereof. Suitable magnet elements have been found to have a magnetization (M) in excess of l0 gauss per gram. magnetization being a measure of the magnetic field intensity of the material from which the particles are prepared. Hard barium ferrite has a magnetization of about 70 gauss/gm and gamma iron oxide has a magnetization of about 50 gauss/gm. Also. it has been found that suitable LII (all

magnet elements should have a magnetic coercivity (defined as the opposite sign field necessary to reduce the magnetization to zero) greater than the magnetic field (H) applied to cause physical movement ofthe element. Magnetic fields of about to about 600 oer steds and higher have been used to move the particles. Hard barium ferrite has a magnetic coercivity ofabout 3000 oersteds and the gamma iron oxide has a magnetic cocrcivity of about 300 oersteds. Magnet elements having a magnetic coercivity less than about l00 oersteds have been found not to be particularly suited for use in the invention because application of external magnetic fields sufficiently strong to move the elements causes demagnetization.

The size of the magnet elements will vary over a considerable range depending upon the coatable particulate material and upon the particular substrate being coated. As previously stated, the mass of the magnet elements being used should be at least twice the mass of the particulate material being coated. Typically. the size ofthe magnet elements will vary between 1 micron in maximum extent to about several hundred microns or more. The size ofthe magnet elements should be sufficiently small to enter any openings or perforations in the article being coated. if it is desired to coat the inner surface of such openings.

The amount of magnet elements used with the coat able particulate material will also vary depending upon the coatable particulate material being used and upon the substrate being coated. Functionally stated. the total mass of magnet elements is that sufficient to cause the coatable particulate material to impinge upon the surface of the substrate being coated and to provide a coating thereon. Since the magnet elements should be at least twice the mass of the coatable particulate material. the total mass of the magnet elements will likewise be at least twice the mass of the coatable particulate material. Usually an excess of the amount of coatable particulate material desired to be coated on the substrate is used or eventually added during the coating operation.

For some applications. other substances may be used with the magnet elements coatable particulate material mixture. For example. with some substrates it may be desired to have the additional abrasive action of a suitable abrasive material in the mixture to provide a smooth clean finish to the surface being coated. Addi tionally. the mixture may contain hard dense particles such as glass beads. metal shot. ceramic beads and the like to aid in hammering the particulate material onto the surface of the substrate.

Of particular advantage. is the fact that two or more different coatable materials can be coated simultaneously by the process of the invention. This feature permits the simultaneous coating of two metals or of a non-metal and a metal together in composite layers. In this manner. composite layers can be made from any of a wide variety of starting mixtures.

The process of the invention is generally carried out under normal atmospheric conditions. however. for some materials (either coatable materials or substrates) it may be desirable to coat in an inert atmosphere such as dry nitrogen. argon. or helium. or to carry out the entire operation in a vacuum or near va cumm. For example. when utilizing magnesium powder as the coatable particulate material. it is preferred to carry out the process in a dry inert atmosphere. Addi- 5 tionally. while it is generally unnecessary. various coating additives or promoters may also be utilized in the process. Such materials may provide a more uniform coating for some coatable particulate materials and for erating device and barium ferrite magnetic particles The rotating magnetic field generating device. originally the stator of a c horsepower electric motor. was a ring-like structure having a 5.5 inch outer diameter some substrates Such additives and promoters are well 5 d u 2 8 i h inner diameter with windings f d f known in the mechanical plating art and useful in the insulated copper wire f i a two l Single phase present process arrangement Another remarkable advantage of the invention is A 500 m] glass beaker to Contain magnet theft subsu'ales to be Coated do not generally ements and the aluminum particles was situated within quire a clean surface. ln other words the substrate may it) the Opening f the Stator d ib d above. A i h have f layer of lush Sca|e- Paint grease I inch X 0.0[0 inch strip of copper to be plated was n Subjected the magnet": pamCle/Coumble held on the wall of the beaker by a strip of double faced particulate material mixture and yet be coated with a pressure sensitive tape. The magnet elements Werc ban P layer Extremely lhlck layer? of Surface ium ferrite speaker magnets which had been crushed to tamination are preferably removed prior to commence- 5 provide a particle Size which passed through a 5 mam f Coaung to Shorten the amount lequlred Standard Screen mesh size of [2 and were retained on to acheve a 40 mesh (approximately 0.42 mm). The barium ferln some Instances the magnet elemfims fhemselvfis rite particles had been previously magnetized by brief may actually coat onto the substrate along with the maexposure in d I I 000 gauss magnetic fie|d terial nominally being coated. lfsuch a situation occurs 30 The powdered aluminum was that 50M by US and is not desired, magnet elements encased in a pro- Bronze Powder Company as Venus Aluminum POW tective shell such as a hard polymeric resin coating may def Atomized N0 610 medium mesh having a particle be used A typuial Cmmng of polyurethane: size of about microns and a bulk powder density of Substrates which can be coated or plated in accor- 1.0 g/CC About 2.5 grams of powdflred Iuminum were dance with the present invention include any hard ma- 35 used terial Such materials include metals. alloys, wood, Coating was accomplished by energizing the rotating g Q I a gl a s sugstmtes may field generating device to an operating current of IO i 5 g u m 0 t redds sharp amps for a period of one hour. A 3 mil thick coating r the A f the surface of x having a matte finish uniformly covering the exposed such an article is in communication with the coatable 0 Surface article was produced particulate material and magnet elements. it will be Coated,

The following examples illustrate the invention. EXAMPLE 2468 EXAMPLE I 35 Utilizing the device described above the substrate Powder d aluminu 8 Coated d pp shown below was coated with the coatable particulate strate surface by utilizing a rotating magnetic field gent i l Shown in th t ble Ex No. Substrate Coating Material 2 aluminum aluminum (Same as Ex. I 3 anodized aluminum 4 stainless steel 5 nickel h copper 7 titanium 8 tiluminizcd stecl 9 31)} stainless steel Ill zinc l 1 magnesium I2 430 stainless steel l3 glass l4 ceramic l5 nylon l6 polystyrene l7 polytctrafluoroclhylenc l8 polycarbonate l9 styrene polymer 2(I polyethylene 21 aluminum barium ferrite (Magnets of alloy Ex. ll 22 anodized aluminum 33 steel 24 nickel 25 copper 26 titanium 27 :iluminizcd steel 28 /inc 2) magnesium 3!) 430 stainless steel 31 glass 32 ceramic 33 nylon 34 polystyrene 35 polytetrafluoroethy lene 3h polycarbonate barium fcrritc (Magnets of alloy E. l I 37 styrene polymer -Continued E\ No. Substrate (outing Material 38 polyethylene 39 aluminum tin I micron) 40 anodized aluminum 4] stainless steel 42 nickel 43 copper 44 titanium 45 aluminized steel 4-6 303 stainless steel 47 line 48 polystyrene 49 aluminum lead (6 micron) 50 steel 5| nickel 52 copper 53 titanium 54 aluminized steel 55 303 stainless steel 56 Zinc *7 aluminum line 14 micron) 58 anodized aluminum 59 steel 6O nickel 6| copper (i2 titanium 63 aluminized steel 64 303 stainless steel 65 IlTlC 6b pol styrene 67 aluminum cadmium l7 micron) 68 anodized aluminum (19 steel 7t) nickel 7] copper 72 titanium 73 uluminized steel M 303 stainless steel '75 zine 76 aluminum eopper (8 micron) 77 anodized aluminum 78 steel 79 nickel Kl) copper lsl titanium N2 aluminized steel H3 303 stainless steel K4 430 stainless steel N5 glass so aluminum graphite N7 anodized aluminum NR steel i4) nickel ll copper It titanium l2 aluminized steel 93 303 stainless steel )5 aluminum molybdenum LllSUlfiCll.

(microsize powder) 16 anodized aluminum 97 steel '98 nickel 9*) copper loo titanium IHI aluminized steel HIZ 303 stainless steel Hi3 43H stainless steel Hi4 aluminum indium (200 mesh) Hi5 anodized aluminum Hi7 nickel 10% copper ill) titanium lltl alumini7etl steel l l l 303 stainless steel I I2 aluminum tantalum 1-4.8 micron) 1 l3 anodized aluminum 1 I4 steel 1 l5 copper l in titanium l l? aluminized steel H8 303 stainl I19 43 stainle.

E20 aluminum chromium l2! anodized aluminum 12: steel I 23 nickel l 24 copper titanium I26 aluminizetl steel 303 stainless steel Lontinucd Ex. No. Substrate Coating Matcrial I28 430 stainless steel l2) aluminum tin (50)!- lcad (50) I30 anodized aluminum 13] steel l3: nickcl I33 copper l 34 titanium 135 aluminized steel 136 303 stainless steel 137 line l 38 aluminum polytetrafluorocthylenc I39 anodized aluminum I40 stecl l4l nickel l42 copper l43 titanium 144 aluminizcd steel I45 303 stainless steel I46 430 stainless steel I47 aluminum silver I48 steel I49 nickel I50 copper l5l aluminizcd steel I52 yinc I53 ABS plastic I54 polystyrene I55 aluminum gold [23 micron) I56 anodized aluminum I57 steel I58 titanium 159 aluminizcd stccl I60 303 stainless steel lol ceramic lhZ nickel I63 molybdenum nickel (0.2'71 max.

200 mesh 2.0% max. r

325 mesh) l64 aluminum magnesium (-400 mesh) I65 glass 16h stccl aluminum/zinc alloy 16'] steel tungsten carhide/ cobalt alloy (325 mesh) in! aluminum iron (325 mesh) The degree of adhesion of each coating tabulated 40 elements, and the aluminum powder are all described above was qualitatively measured by a tape test." using a /4 inch wide and 1 i2 inch long strip of pressuresensitive adhesive tape sold under the trade designation Scotch Brand Magic Mending Tape" by the 3M Company. For the test. one-half inch of the strip adjacent one end was adherently bonded with finger pressure to the coated surface. and then the free end of the strip was doubled hack on itself at 180 and slowly pulled away to completely remove the tape from the article. An adequately adhered coating was one which did not split or fail under the test. The coatings in all of the examples remained intact when subjected to the tape test just described. except for those of graphite and molybdenum disulfidc (Examples 86-103) which. of course, would not be expected to do so. since they have a weakly cohesive nature.

EXAMPLES l69-l74 The following examples show the effective weight ratio of particulate material to magnet elements useful in the invention. For each example. a copper piece was attached inside the container consisting of an 8 ounce paper drinking cup which also contained I00 grams of magnet elements. The rotating magnetic field generating device was operated at l l amperes for 30 minutes in each case. The amount of aluminum powder used for each example is shown in the table below. The rotating field generating device. the copper pieces. the magnet in Example l.

After the coating was accomplished. the coating thickness and weight were measured. Results are as follows:

Ex. Powder Coating Thickness Coating Weight No. Weight (g) (mill (gl l6) 2 .25 .Utllfi 50 l70 5 .26 .0022 l'fl l0 27 .0025 H2 20 .25 .0016 l73 4.0 .22 .0009 174 I5 .0009

As can be seen, the efficacy of coating is reduced somewhat if the weight of particulate material is greater than about l/lOth the weight of the magnet elements, indicating that it is preferred to maintain a relatively small amount of particulate material with respect to the magnet elements.

What is claimed is:

l. A process for coating particulate material upon the surface of a substrate comprising:

exposing said substrate surface in a confined volume to a plurality of disconnected and independently movable small permanent magnet elements and said particulate material. and

establishing within said volume. in addition to the magnetic field of said permanent magnet elements, a magnetic field varying in direction with time and of sufficient intensity to impart movement to the magnetic elements to cause the particulate material to impinge upon and to coat said exposed substrate surface. 2. The process of claim 1 wherein said established magnetic field rotates about a central axis.

3. The process of claim I wherein said magnetic elements have an electromagnetic field of at least about I gauss,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 9 ,90

DATED July 1, 1975 INVENTOR(S) 1 William R. Lovness it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown betow:

Col. 3, line 59, change "(BaO.6Ee 0 to -(BaO.6Fe O Col. 10, Example 172, change "2.0" to --20-.

Col. 10, Example 173, change 4.0" to ----H0--.

Signed and Scaled this A nest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer ('ummissmm'r nj'lan'nls and Trademarks 

1. A PROCESS FOR COATING PARTICULATE MATERIAL UPON THE SURFACE OF A SUBSTRATE COMPRISING: EXPOSING SAID SUBSTRATE SURFACE IN A CONFINED VOLUME TO A PLURALITY OF DISCONNECTED AND INDEPENDENTLY MOVABLE SMALL PERMANENT MAGNET ELEMENTS AND SAID PARTICULATE MATERIAL, AND ESTABLISHING WITIN SAID VOLUME, IN ADDITION TO THE MGNETIC FIELD OF SAID PERMANENT MAGNET ELEMENTS, A MAGNETIC FIELD VARYING IN DIRECTION WITH TIME AND OF SUFFICIENT INTENSITY TO IMPART MOVEMENT TO THE MAGNETIC ELEMENTS TO CAUSE THE PARTICULATE MATERIAL TO IMPINGE UPON AND TO COAT SAID EXPOSED SUBSTRATE SURFACE.
 2. The process of claim 1 wherein said established magnetic field rotates about a central axis.
 3. The process of claim 1 wherein said magnetic elements have an electromagnetic field of at least about 100 gauss.
 4. The process of said claim 1 wherein said magnetic elements have a magnetization of at least 10 gauss per gram.
 5. The process of claim 1 wherein said magnetic elements are barium ferrite.
 6. The process of claim 1 wherein said particulate material is powdered aluminum.
 7. A coated substrate comprising a substrate having a surface coating of a multitude of flattened particles adhered to said surface, said coating containing magnet elements. 